Bragg grating technology has been a subject of intensive research for the last decade, especially for its application in the telecommunication industry. Although the optical performance of Bragg gratings is high and extremely attractive for many applications, some problems still persist such as the well known ripples in the group delay spectrum, especially observed in gratings for dispersion compensation.
Various approaches have been used to further improve the optical properties of Bragg gratings, and in particular minimize the group delay ripples (GDR). Bragg gratings are usually made by photoinducing in an optical fiber, or other photosensitive medium, a diffraction pattern, produced by actinic radiation projected through a phase mask. An important source of defects in the resulting grating is the mask itself which usually contains phase errors due to its own manufacturing process. Efforts have therefore been made to improve the quality of phase masks, and therefore reduce the defects in the resulting Bragg gratings. For example, KOMUKAI et al (“Group delay ripple reduction and reflectivity increase in a chirped fiber Bragg grating by multiple-overwriting of a phase mask with an electron-beam,” IEEE Photon. Technol. Lett., vol. 12, pp. 816–818 (2000)) discloses a step-chirped phase mask made by overwriting a pattern at the same place on a substrate several times, using an electron-beam in a continuous movement approach. The same authors have also suggested another strategy for fabricating a step-chirped phase mask using a raster scan-type laser-beam writing system (T. Komukai, T. Inui, M. Kurihara, and S. Fujimoto, “Group-Delay Ripple Reduction in Step-Chirped Fiber Bragg Gratings by Using Laser-Beam Written Step-Chirped Phase Masks,” IEEE Photon. Technol. Lett., vol. 14, pp. 1554–1556 (2002)). It is notable that, as they modify the masks themselves, both these methods will only correct for mask-related, systematic defects in the resulting gratings.
Instead of trying to minimize or eliminate phase errors in phase masks, another approach is to minimize their effect on the resulting grating, by a proper characterization of the origin of systematic defects in the gratings produced by a given system, and an appropriate feed-back on the fabrication process. Such a technique is for example shown in published U.S. patent application no. 2003/0059164 (STEPHANOV et al.) and A. V. Buryak, and D. Y. Stepanov, “Correction of systematic errors in the fabrication of fiber Bragg gratings,” Opt. Lett., vol. 27, pp. 1099–1101 (2002). STEPHANOV et al. discloses a method for compensating for phase errors in a Bragg grating by first making a test grating, and then measuring its spectral characteristics. These characteristics are used to reconstruct the actual design of the grating, preferably using a Layer Peeling Method. A compensated design is obtained by comparing the reconstructed design to the theoretical structure, for example by direct subtraction of deviations therebetween, and the compensated design is finally used to make subsequent gratings using the same optical system. Of course, only systematic defects inherent to the particular optical system used to make the gratings will be compensated for by this method.
Another alternative is to apply a post-treatment to the photoinduced grating. Post-correction of Bragg gratings was already proposed for other purposes, such as tuning the dispersion or other optical characteristics of the grating (see for example K. O. Hill et al. “Chirped in-fibre Bragg grating dispersion compensators: Linearisation of dispersion characteristic and demonstration of dispersion compensation in 100 km, 10 Gbit/s optical fibre link,” Electron. Lett., vol. 30, pp. 1755–1756 (1994); and K. O. Hill et al. “Chirped in-fiber Bragg gratings for compensation of optical-fiber dispersion,” Opt. Lett., vol. 19, pp. 1314–1316 (1994)). When applied to the correction of defects, this approach has the advantage of alleviating both systematic and non-systematic errors. Referring to published U.S. patent application no. 2003/0186142 (DESHMUKH et al.) and M. Sumetsky et al, “Reduction of chirped fiber grating group delay ripple penalty through UV post processing,” Tech. Dig. Post deadline papers, OFC'2003, PD28, there is shown such a technique. The Bragg grating is photoinduced in a photosensitive medium, and a test beam is launched in this medium during, or at the end of the writing process, to optically characterise the grating. The collected data is used to calculate a post-correction to the grating, using a correction algorithm based on a simple solution to the inverse problem relating the measured GDR vs. wavelength to the desired change in Bragg wavelength vs. position. In this approach, only the low frequency part of the GDR is compensated for, which means that the technique only corrects for large defects in the Bragg grating, that is about 10 mm or higher. Several iterations can be made to optimize the benefit of this technique; after the correction is applied, the optical properties of the grating are again measured, and a new correction calculated, this process being repeated until a satisfying suppression of the GDR ripples is achieved.
In spite of all the above-mentioned work, there is still a need for defects-correction techniques having improved optical performances. In particular, there is a need for a technique that takes into consideration any or all of the types of defects found in Bragg gratings and appropriately compensates therefore.